medwireNews: Body mass index (BMI) at time of colorectal cancer diagnosis has a U-shaped relationship with the risk of mortality, suggests a study, adding support for an obesity paradox in this patient population.

“Despite scientific evidence showing that exercise in patients with cancer should be encouraged, findings suggest that recommendations for weight loss in the immediate postdiagnosis period in patients with CRC who are overweight may be unwarranted”, the authors comment in JAMA Oncology.

Candyce Kroenke, from Kaiser Permanente in Oakland, California, USA, and co-authors report a nonlinear relationship between BMI at diagnosis and all-cause mortality for 3408 men and women who were diagnosed with stage I to III colorectal cancer between 2006 and 2011.

Compared with patients who had a low–normal weight at diagnosis (BMI 18.5 to <23.0 kg/m2), all-cause mortality was higher in underweight patients (BMI <18.5 kg/m2) and those with class II or III obesity (BMI ≥35 kg/m2), with hazard ratios (HRs) of 2.65 and 1.33, respectively.

By contrast, patients with high–normal weight BMI (23 to <25 kg/m2), low–overweight BMI (25 to <28 kg/m2) or a high–overweight BMI (28 to <30 kg/m2) had a reduced risk of all-cause mortality, with HRs of 0.77, 0.75 and 0.52, respectively.

And there was no significant difference in the risk of all-cause mortality between low–normal weight participants and those who had a class I obesity BMI (30 to <35 kg/m2). Further analysis pointed to patients with a BMI of 28 kg/m2 as having the lowest risk of mortality, the team adds.

The researchers also found a similar relationship between BMI at diagnosis and colorectal cancer-specific mortality, as well as between postdiagnosis BMI and mortality, although they note that class I obese patients also had significantly lower all-cause and cancer-specific mortality in this analysis.

While acknowledging the possibility that excess adiposity might confer risk reduction by impacting biological mechanisms or treatment response, the authors of an accompanying editorial highlight potential selection bias and potential confounders that could explain the apparent paradoxical relationship, such as weight loss before diagnosis and colorectal cancer treatment received.

“Although we appreciate that the interpretation of the association between BMI and mortality in patients with cancer is considerably more challenging than that between BMI and incidence cancer risk, we have a duty to appropriately convey this complexity to our patients and populations”, write Andrew Renehan and Matthew Sperrin, from the University of Manchester in the UK.

They therefore conclude: “These findings should not alter current clinical practice or recommendations.”